Rewrite README for v2.0.0

[apermo]

Complete README rewrite for the rebranded plugin.

• New title and description referencing the Surf ecosystem
• Ecosystem links (CLI, Chrome, planned extensions)
• Installation via wordpress.org, Composer, and manual
• Configuration: database settings, PHP filter API, `.surf-links.yml`
• Feature overview with screenshots
• Shell completions / CLI bridge (`surf add-to-chrome`, future `surf add-to-wp`)
• PayPal donate link
• AI disclaimer
• License